This invention pertains to a duplex document copying machine, and more particularly, to such a machine in the form of an automatic document handler, which incorporates a drum for the selective inversion of documents and delivery to a duplex scanning station.
The apparatus of the present invention is directed to a copier of the general type shown in Davis U.S. Pat. No. 4,441,806 issued Apr. 10, 1984 , in which sheet type documents are delivered serially to a simplex scanning station, and moved past the station at a controlled rate. From the simplex station each document is delivered to a duplex drum, in which document guiding elastic transport belts partially encircle the upper surface of the drum. After traversing an upper portion of the drum, the document lead edge is stripped from the drum and presented to a duplexing gate positioned transversely of the drum. If the document has been programmed to be copied on one side only, the gate will be closed when the lead edge arrives, allowing the document to pass directly out of the machine to a collection station. However, if the job was programmed for duplex copying (two-sided originals), the duplex gate will be opened and the document then directed toward the nip of an idler roller in contact with the drum and in combination with guides, is carried to a duplex platen or scanning station for traversing movement past the scanning station. After duplex scanning, the leading edge of the document is carried up circumferentially about the drum, where it rejoins the simplex document path at a vertical tangent on the duplex drum. After transport across the top half of the drum, the document leading edge is again stripped and presented to the duplex gate. The gate is now closed, directing the document to a document receiving tray as in simplex operation.
In U.S. Pat. No. 4,441,806, a document diverter gate arrangement is disclosed, by means of which the document is either stripped from the drum or is permitted to traverse about the surface of the drum to the duplex scanning station. However, since the surface of the drum is coated with a white elastomer blanket, to provide a white background at the duplex window, and since the proper operation of the automatic document handler requires optical pickups to detect the presence and location of the document with respect to the drum, it has proved difficult to detect the document at the gate, due to lack of contrast. Attempts to calculate the position of the document taken from a measurement prior to the document entering the drum has proved to be difficult, in view of variations in the times taken by different documents to arrive at the duplex station. This is believed to be primarily due to the fact that documents must be handled of substantially differing weights, and the differences in weight result in small differences in the actual distance traveled from a nominal distance, thereby creating registration errors. Accordingly, a need has arisen for a document lead edge detector to be placed substantially closer in the travel path of the document to the duplex station for the proper programming of duplex copying.